The present invention relates to a wide-band programmable tachometer, and speedometer/odometer apparatus (hereinafter will be referred to as only the meter) to detect the rotational speed of an output shaft of an engine in a vehicle and to calculate the corresponding speed/mileage thereof. More particularly, the present invention relates to a tachometer which is adapted to indicate an extremely wide range of rotational speeds of an engine, ranging from the hundreds of rpm's (revolutions per minute) of an automobile to the hundreds of thousands of rpm's of an airplane.
Referring to FIG. 1, in conventional tachometers 100, a converting means 110, which converts a mechanical motion of an engine into an electrical signal, is connected to an actuating means 120 with a first micro-adjustment selecting means 121. The rpm value of the engine is shown on the dial of a display means 130 driven by the actuating means 120 after the electrical signal is processed therein. The frequency of the electrical signals sent into the currently available actuating circuit 120 are limited within an appropriate operable range; for example, they could be from 133 Hz to 265 Hz for a low-frequency actuating circuit 120. Therefore, various kinds of tachometer 100 are required to adapt to the different ranges of operating speeds of engines for automobiles, airplanes, and other machines. As a result, the tachometers for high frequency (about 1.2 kHz to 19.9 kHz) and low frequency (133 Hz to 1,248 Hz) are developed to be respectively employed to meet different requirements.
There are several methods to convert the motion of an engine into an electrical signal which corresponds to the rotating speed of the engine, and can therefore be used to indicate the rotating speed, for example, by way of pulse generator-type sender, magnetic proximity, alternator-type sender, or standard ignition coil. The first micro-adjustment selecting means 121 comprises a plurality of (for example, six) switches to selectively suit the different ranges of frequency response of the actuating circuit 120 with respect to the electrical signals, wherein the ON/OFF states of the six switches give 2.sup.6 (64) combinations. With the 64 combinations, the operating range for the frequency range within 133 Hz to 265 Hz can be divided into 64 different periods, in which each period has the range of (165-133)/64=2.06 Hz. It should be noted that the ON/OFF states of the switches for the micro-adjustment selecting means 120, which is pre-set before the tachometer 100 is fabricated, are obtained by calculating a full-scale frequency of the electrical signal. Based on the full-scale frequency of the electrical signal, the scale on the dial of the display means is adjusted before the tachometer 100 is fabricated.
It is the purpose of this present invention, therefore, to mitigate and/or obviate the above-mentioned drawbacks in the manner set forth in the detailed description of the preferred embodiment.